Remove lock around tokio-rt handle (#36762)

It seems sub-optimal to to sequentialise execution by grabbing a lock,
each time we want to spawn a task onto the tokio runtime. We don't need
the lock either, so it makes sense to just remove it, which also
simplifies a bunch of the using code.

Testing: Covered by existing tests

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

components/net/resource_thread.rs

@@ -771,7 +771,7 @@ impl CoreResourceManager {
             _ => (FileTokenCheck::NotRequired, None),
         };
 
-        HANDLE.lock().unwrap().as_ref().unwrap().spawn(async move {
+        HANDLE.spawn(async move {
             // XXXManishearth: Check origin against pipeline id (also ensure that the mode is allowed)
             // todo load context / mimesniff in fetch
             // todo referrer policy?